1. Field of the Invention
The invention relates generally to an information reproducing apparatus, and more particularly to an information reproducing apparatus and an information reproducing method for performing decoding using PRML (partial response and maximum likelihood) techniques.
2. Description of the Related Art
In recent years, information recording/reproducing apparatuses for performing recording/reproduction with a recording medium, for example, an optical disk such as a DVD (digital versatile disk), have enjoyed widespread public use. This involves growing demands for implementing even higher recording densities by use of various techniques. In response to such demands, for example, PRML techniques are used as information recording/reproducing techniques for recording media such as optical disks.
In a PRML signal communication technique, a reproduction signal corresponding to optical-disk containing information detected by a pickup head is partial-response (PR) equalized by an equalizer or the like, an equalized signal is thereby obtained, the equalized signal is maximum-likelihood decoded, and the optical-disk containing information is thereby reproduced.
A maximum likelihood decoder (ML decoder) calculates an Euclidean distance between the equalized signal and an ideal signal, and decodes the signal into a bitstream that outputs an most likely ideal signal. In PRML signal processing, a bit error rate of a bitstream decoded by the maximum-likelihood decoder much relies on the equalized signal level. As such, control of the equalized signal level, specifically, the signal amplitude, is a problem to be resolved.
As a prior art related to the above, a digital device has been proposed (as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-44998). In the proposed digital device, an amplitude control circuit for controlling the amplitude of an equalized signal performs amplitude control in accordance with the difference between a signal of an equalizer and a target signal used to perform maximum-likelihood decoding.
In the prior art, however, in PRML signal processing, errors can occur more easily as the Euclidean distance between correct and error bitstreams is shorter. Generally, such a bitstream includes contiguous bits conforming to a minimum run-length constraint, such as 2T. A deviation may be caused between the reference level and the level of the equalized signal produced by a minimum run-length sequence of, for example, 2T. Thus, according to the prior art, a deviation is caused between the reference level and the peak level of the equalized signal in shortest contiguous bits of, for example, 2T. This increases the probability of causing bit errors in decoding processing by a maximum-likelihood decoder in a rear stage.